1. Field of the Invention
The present invention relates generally to an internal combustion engine valve train and more specifically to a poppet valve control arrangement which features a pair of rocker arms which are arranged in a scissor-like manner and wherein one of the arms is used to lift the valve while the other is used to close the same and wherein a cam arrangement is provided by which the valve timing can be variably adjusted in accordance with vehicular driving conditions.
2. Description of the Prior Art
Mechanisms are known in the art of internal combustion engines for adjusting the timing of the induction and exhaust valves in a manner so as to best meet the particular conditions under which the engine is being operated. Such systems are generally adapted to be operable to provide the most efficient valve timing for slower or faster engine ranges.
An example of one such prior art valve timing adjustment system may be found in JP-A-55-96310.
In the above mentioned mechanism, the cam for lifting the poppet valve is formed so as to have a profile which changes along the longitudinal axis of the cam shaft. Thus, the cam face defines a compound curve and may be referred to as a compound cam.
The cam shaft, on which the compound cam is formed, is axially movable along its rotational axis so as to facilitate the adjustment of the valve timing. As the cam shaft on which the compound cam is moved along its rotational axis the profile of the peripheral portion of the cam which comes into engagement with the cam follower changes. In this manner both the valve lift and the valve timing can be changed.
For example, it may be deemed desirable to cause the valve to be opened to a lesser degree and for a shorter period of the engine cycle during low speed/low load operation of the engine in order to provide greater fuel efficiency. On the other hand, it may be desirable to operate the cam into a position wherein the valve stroke and the cycle duration of valve opening may be lengthened during the high speed operation of the engine in order to provide the maximum power output and to improve the intake and exhaust efficiency.
In the above mentioned system problems exist in that, with most conventional poppet valve opening mechanisms the valve is opened by a cam and is returned to the closed position by a spring. Because of this spring, a large force is generally required in order to open the valve against the closure pressure of the valve closure spring. Therefore, in order to move the compound cam along its axis against the friction and pressure exerted thereon by the valve closure springs, a rather large motive force is required.
It is particularly difficult to move the cam shaft along its axis against the increased friction that is generated while the valves are in the open positions, since it is then that the maximum degree of pressure is being exerted on the cams through the cam followers by the valve closure springs.
Thus, in order to assure that the cam shaft can be moved along its axis quickly enough so as to adjust the valve timing in such a manner as to keep up with the rapidly transient changes which occur when operating the engine of an automobile, the motive force to drive the cam shaft along its axis for adjusting the compound cam becomes even larger.
In view of the above problems related to reducing the motive force required for driving the cam shaft along its axis it had been proposed during the development of the present invention, to actuate the cam shaft to move during the moment of least friction between the cams and the cam followers. This moment being that when the portions of the cams which are at the smallest distance from the axis of the cam shaft are in contact with or slightly disengaged from the cam followers.
Unfortunately, in engines having a large number of valves, the length of time for which these smallest portions of the cams are all in engagement with the cam followers, i.e. when the valves are all closed, is extremely short and timing the movement of the cam shaft to coincide with this moment of least friction is exceedingly difficult.
Another problem encountered in the above system is that the cam surface is formed with sections having varying angles with respect to the axis of the cam shaft. As a result, if the cam follower is mounted solidly on the tip of the rocker arm or the valve stem, there is point contact between the cam follower rather than the line contact conventionally sought after. This type of engagement results in a high concentration of friction at the contact point resulting in extremely rapid wear on the cam follower and on the cam.
On the other hand, attempts have been made to make line contact between the cam follower and the cam by providing a swiveling tip on the cam follower that can pivot so as to remain as nearly flat against the varying contours of the cam as possible. The provision of such a swiveling tip however adds weight as well as making the cam follower prohibitively complex and expensive.
Attempts have also been made in the prior art as in shown in JU-A-60-3412 and JP-A-60-32910 to provide a mechanism whereby the induction or exhaust valve of an internal combustion can be made to open and close stably when the engine is running very fast. In order to achieve this aim, the above valve opening mechanisms comprise a cam shaft on which a opening cam and a closure cam are fixedly mounted in a side by side relationship.
A rocker shaft supports an opening rocker arm and a closure rocker arm. The opening rocker arm is provided with an adjust screw via which the valve clearance can be adjusted.
A poppet valve, which can be either an inlet valve or an exhaust valve, has a stem the top of which is provided with a retainer. The retainer is formed with a radially extending flange at the bottom thereof, which is arranged to be engaged by the leading or outboard end of the closure rocker arm. The end of the rocker arm is formed with a U-shaped recess which defines two bifurcate finger members. These fingers extend on either side of the valve stem and engage the bottom of the retainer.
In operation, as the cam shaft rotates, the opening and closure cams rotate to positions wherein the high and low lift portions thereof engage the opening and closure rocker arm followers. This induces the opening rocker arm to rotate in a direction which brings the end of the adjust screw into engagement with the top of the valve stem and applies a force which tends to move the valve head off the valve seat. Simultaneously, the closure rocker arm is rendered rotatable in the same direction as the opening rocker arm, and thus relaxes the force which tends to bias the valve head into engagement with the valve seat.
Accordingly, the valve head is moved to an open position.
As the cam shaft continues to rotate, the low lift and high lift portions of the opening and closure cams come into contact with the followers of the opening and closure rocker arms, respectively. This causes the closure rocker arm to rotate in a direction which forces the lead end thereof against the lower face of the retainer and produces a force which moves the valve upwardly and which permits the opening rocker arm to be rotated in the same direction. The valve is thus moved until the valve head engages the valve seat and closes the valve.
In another arrangement, the opening rocker arm is arranged to engage a flanged retainer at its leading end and to have a follower formed thereon at a location distal from the axis about which it is pivotally mounted. The closure rocker arm is pivotally mounted on the same rocker shaft as the opening one, and provided with a clearance adjust screw which is arranged to engage a portion of the opening rocker arm located proximate the shaft on which the two rocker arms are pivotally mounted.
With this arrangement, as the cam shaft rotates essentially the same type of push-pull type of operation as in the previous arrangement occurs and the valve is opened and closed.
For further disclosure relating to such types of arrangements, reference can be had to JU-A-60-3412 and JP-A-60-32910.
However, the above valve train driving mechanisms have the disadvantages that the cam clearances for both the opening and the closure rocker arms must be set very precisely and the clearance adjuster mechanisms are both complicated to build and difficult to adjust. This makes the valve train expensive to manufacture and maintain.
What is more, with the passing of time, the valve heads and seats tend to undergo localized wear and/or deterioration. This, as is well known, leads to the loss of sealing by the valve and invites loss of efficiency and proper engine operation. These arrangements have also suffered from the drawback that both of the clearances between the opening and closure rocker arms and the portions of the valve and the retainer which they engage, is difficult to set and maintain. Accordingly, these type of arrangements have tended to be noisier than the conventional spring loaded types.
In view of the above problems the inventors of the present invention proposed a valve driver mechanism in which the clearance adjusting operations were simplified in JU-63-203521.
In the latter application, a rocker arm mechanism was proposed wherein the opening and the closure rocker arms are pivotably connected to each other by means of a pin and one of the rocker arms is pivotably connected to the cylinder head by a ball and socket pivot connection so as to pivot on an axis that is parallel to the pivoting axis defined by the connection between the opening and closure rocker arms. In this mechanism, by adjusting the position of ball of the ball and socket joint, the valve clearance for both the opening and closure cams can be carried out in a single simple operation.
The latter device while facilitating easy adjustment of the rocker arm clearances has the drawbacks that, since the connection between the cylinder head and the rocker arm unit is formed by a single ball joint, the rocker arms have a tendency to sway side to side about the ball and socket joint and or to tilt out of the proper rocking plane. Thus the later rocker arm pivot mechanism suffers the disadvantage of providing little or no lateral or vertical stability to the rocker arms.
These tendencies of the rocker arms to tilt and vibrate lead to accelerated wear of the cam and cam follower surfaces and widen the clearances resulting in noisy valve opening operation. This was particularly problematic when the engine is running fast and even a small magnitude of vibration tended to create a great deal of noise. What is more, this lateral and vertical play in the rocker arms tends to disrupt the valve timing and valve seating and makes precise adjustment of the valve timing impossible.
Thus, the complexity and the resulting expense of building and as well as the difficulty of maintaining the valve clearance adjustment of the above mentioned `desmo-dronic` type valve control rocker assemblies, as well as the magnitude of noise they create due to the resulting cam clearances has somewhat restricted their field of application to high performance and racing type engines in which noise and labor intensive maintenance are taken as a matter of course.
In such engines, since they are generally designed to be operated in a range referred to as the power band, idling and low range fuel efficiency is generally not a major consideration. On the other hand, weight and compactness are a high priority design factors. Therefore the application of an adjustable compound cam shaft and the associated weight and bulk in an engine having a desmo-dronic type valve control rocker assembly has not to date been proposed due to the conflicting design objectives.
On the other hand, the attempt to provide a simplified rocker arm arrangement by providing the adjustable ball and socket type pivot seemed to hold the promise of bringing the desmo-dronic type valve control arrangement having the closure cam as a replacement for the valve closure spring into the realm of the middle class of automotive engine. Unfortunately, this raised the problems of lateral and vertical pivoting instability in the rocker arms.
What is more, due to this lateral and vertical instability, the application of the ball and socket type rocker arm pivot arrangement in association with a cam shaft movable along its axis posed apparently insurmountable complications in that the movement of the cam shaft along its axis would drive the ends of the rocker arms to sway side to side with the result that the cam followers would not change positions on the compound cams as intended. This also had the result of causing the rocker arms to exert a large lateral force on the valve stems that could cause a great deal of wear and/or even bend them.